Our recent studies of bacteriophage morphogenesis have focused on the assembly of T4 capsids. We have investigated the roles of several phage proteins in T4 head assembly and in the in vivo packaging of DNA into preformed phage heads. In our studies, we have principally utilized three basic approaches: First, to better understand functional interactions among various phage proteins during capsid assembly, we have isolated and characterized new T4 mutants that permit virus production in the absence of viral proteins that are normally essential to these processes. We have analyzed the physical and chemical properties of such viruses and genetic mapping of the mutations that bypass requirements for specific proteins. These studies have produced new information about the interactions of various phage proteins and about the functions of different proteins in capsid morphogenesis. Second, to determine which phage proteins are needed to package DNA into capsids, we have performed temperature shift experiments with cells infected by T4 particles containing appropriate combinations of cold-and temperature-sensitive mutations. In this way we have been able to block the functions of specific T4 proteins and to determine whether the absence of these functions in turn blocks packaging of DNA into preformed capsids. Third, to gain additional information about the roles of various proteins in capsid assembly, we have used the electron microscope to characterize the structures produced following infection by different T4 mutants.